How can I make better UVs for my photogrammetry models?
If you have issues editing your UV layout in an external program, you might keep a small margin and increase the exported UV layout's resolution instead:



Also, if your output render engine supports multiple UV sets for one object, you might just make several UV unwraps which each gives more pixels to different areas. You could have one UV Map for the house for example, and one for the other parts.



For rendering in Cycles, you will just have to use the Input → UV Map in order to specify which UV Map to use in your shader. You can have one node for each UV Map you need:



Or play it the "Divide and Rule" way: 
Just split your mesh into different smaller objects and give them what they need in UVs and textures.
In order to split: go in edit mode, select the mesh you want to split, then hit P → Separate Selection. What you selected will be sent into a new object. Repeat as much as you need.